Optical sensors (Fibre Bragg Gratings (FBG)) have been proposed for many sensing applications. For example a tunable laser or wavelength swept source can be used to read the values on the sensors arranged in serial or parallel combinations.
The centre wavelengths of the bragg reflectors in the fibre sensors change due to the change in temperature or strain experienced by the sensors. An optical wavelength tuneable/swept source (e.g. a tunable laser) can be used to sweep across all wavelengths covering a certain band (e.g. C/L band) and find the reflected wavelength peak location for each sensor response. These wavelength peak movements are related to the measurement source (e.g. strain, temperature, . . . etc). The system used for the measurement of changes on the FBG sensors is referred to as an interrogator system. The system can also be used to characterize optical components (e.g. optical filters).
The tunable laser in the interrogator system must be swept in a linear fashion in order to measure back a linear response of the FBG. In practice the tuneable source is characterized to generate a linear sweep. The interrogator system disclosed in U.S. Pat. No. 7,649,917, assigned to Intune Networks Limited, provides a tunable laser that scans linearly across all segments that are used to generate a quasi linear sweep over a full operational band (e.g. C/L-bands). The quasi linear sweep was based on stitching multiple overlapping mini-sweeps (segments) using a periodic wavelength reference e.g. Etalon to guarantee full coverage. The Etalons used are typically 25 GHz and minimum of 25 GHz was required to have a common Etalon for stitching, which results in a limit on the maximum achievable sweep rate.
A problem with tunable lasers is that due to aging, environmental temperature changes, an extra overlap would be required to improve the margins and guarantee stitching between segments. Also any induced electrical noise in the tunable laser system, the sweep may not stay linear, and mode jumps in addition to instantaneous frequency noise can occur at the output. Other reported interrogators using continuous swept sources use a wavelength reference for frequency tracking as disclosed in US2010/0105148 A1, assigned to Corning Incorporated. However Corning do not address the stitching and overlap issues for tunable laser sources based on quasi continuous sweeps and potential mode jumps that can occur.
A further problem with FBGs interrogated with interrogation systems based on a polarized tunable lasers, is birefringence which leads to polarization dependence frequency shift (PDFS) which would cause a shift in the peak location and error in the measurement. The interrogator system disclosed in U.S. Pat. No. 7,649,917 address this problem by using a polarization switch/controller in the system. However with tunable lasers based on polarized sources and narrow linewidth lasers, passive de-polarizers (e.g. Lyot depolarizer) are not practical due to the long length of fibre required.
It is therefore an object of the invention to provide a system and method to overcome at least one or more of the above mentioned problems.